The present invention relates to a receiving terminal of a discrete multitone (DMT) system, and more particularly, to a device and method for recovering a sampling clock signal during initialization of a receiving terminal of the DMT system.
In general, the DMT system uses a multicarrier to maximize the efficiency of a channel by which data is transmitted. Several carrier-modulation waveforms are overlapped to represent an input bit stream in multicarrier modulation. A multicarrier transmission signal is the sum of N individual subsignals having the same bandwidth ##EQU1## (f.sub.Nyquist is the Nyquist frequency) and center frequencies f.sub.i (i=1, 2, 3, . . . , N). These subsignals or subchannels are quadrature amplitude modulation (QAM) signals. In particular, when data is rapidly transmitted in an inferior transmission line such as a phone line, high-quality services can be rendered by using the DMT system since the transmission speed can be 6 Mbps or higher in applications of asymmetric digital subscriber lines (ADSL), and high-speed transmission is possible depending on the length and characteristics of the transmission path in applications of very high bit rate digital subscriber lines (VDSL). Even in a high-frequency area generating much noise, the system can dispense with a near subchannel, thereby protecting itself against various external damage factors.
The receiving terminal of the DMT system performs the functions of time recovery, filtering, and channel confirmation. The present invention pertains to time recovery and sampling clock signal recovery in the function of time recovery.
Sampling clock signal recovery, which is essential to every communications system, requires that a series of clocks, of a digital-to-analog converter in a transmitting terminal, are recovered in the receiving terminal. Since the DMT system simultaneously performs modulation and demodulation in DMT block units, i.e., inverse-fast-Fourier-transformer (IFFT) or fast-Fourier-transformer (FFT) performing units, during transmission and reception, the DMT system's receiving terminal should have a better clock signal recovering ability than that of a single carrier system.
A conventional sampling clock signal recovering device in the receiving terminal of the DMT system recovers a sampling clock signal, using only a predetermined data transmitted to a predetermined subchannel. Thus, noise of the 64th subchannel is reflected in the recovered sampling clock signal, influencing the sampling clock signal. Though a loop filter in phase-locked-loop (PLL) reduces the influence of noise, phase noise is not sufficiently reflected when the bandwidth of the loop filter is excessively reduced, whereas additive white Gaussian noise (AWGN) increases too much when the bandwidth increases. As a result, it is difficult for a phase locked loop (PLL) to perform accurate tracking.
In particular, in a system for simultaneously demodulating data of all subchannels using a fast-Fourier-transformer (FFT) such as a DMT system, AWGN-induced jitter of its receiving terminal adversely affects the recovery of data of all subchannels, especially data of the subchannel in a high-frequency area, thereby increasing the likelihood of generating many errors during high-speed data transmission.